This invention relates to a method for developing electrostatic images formed on an electrostatic image-bearing member, particularly to a method for development by forming a thin and uniform toner layer on a member for carrying toner (hereinafter frequently called also as "developer" without discrimination) and also to a toner to be used for the method.
In the prior art, the following methods have been known as the method for carrying out development by use of a one-component non-magnetic toner.
One method comprises providing a movable developer-carrying means which carrys, conveys and feeds a developer to a latent image-bearing member, a developer-feeding means and a movable coating means which receives feed of the developer from the developer-means and applies the developer to the above movable developer-carrying means, said movable coating means having a fiber brush for carrying the developer on its surface and contacting the above movable developer-carrying means to apply the toner uniformly to the above movable developer-carrying means at the contacted portion, while moving at higher speed in the same direction as the movable developer-carrying means than the movable developer-carrying means, and approaching the coated layer to the electrostatic latent image portion. Another method comprises providing a rotatable magnetic roller for forming a magnetic brush by attracting magnetic carriers for charging one-component non-magnetic toner particles and a developing roller for developing electrostatic latent images on an electrostatic latent image-bearing member by transfer of the toner particles onto the roller, and developing the electrostatic images, while maintaining a gap between the electrostatic image-bearing member and the developing roller, with the gap being set greater than the thickness of the coated toner layer on the developing roller. Still another method known in the art comprises arranging a developer-carrying member carrying a developer on its surface so as to face an electrostatic image-bearing member, thereby developing electrostatic images on the image-bearing member, wherein the developer accumulated in a developer-storing means below the developer-carrying member is drawn up onto the developer-carrying member, while giving vibration to the developer only at the drawing-up position to activate the developer and form a developer layer to a desired thickness on the surface of the developer-carrying member, thereby developing the electrostatic images on the electrostatic image-bearing member. As the steps after development, similarly as in the electrophotographic method known in the art, the toner developed on the electrostatic image-bearing member is transferred onto a transfer material such as paper, followed by fixing by an energy of heat, pressure, or heat and pressure.
The energy required for fixing is preferably as small as possible. In recent years, as a toner fixable at a low pressure or a low temperature, a microcapsule toner has been proposed, in which a hard resin is applied by coating around the cores of a soft or low melting binder containing a dye or pigment dispersed therein. However, when such a microcapsule is provided for use in the developing methods employing one-component non-magnetic developer as described above, the following difficulties are encountered. That is, in the case where a toner of the prior art prepared according to the crushing method by melting and kneading a binder resin and a dye or pigment, cooling and crushing the mixture, followed by classification, is used for the above developing method, triboelectric charges between the toner and the movable developer-bearing member will increase with the time of friction but, after reaching a certain quantity of charges, the surface charges on the toner are discharged through the dye or pigment exposed at the toner surface or through the tips of projections at the toner surface. Consequently, when the toner according to the crushing method is provided for use in the above developing method, the triboelectric charges of the toner can be maintained constant as the result of equilibrium established between charging of the toner due to friction with the movable developer-carrying member and discharging, whereby the image density of the copy may also be maintained constant. On the other hand, when a microcapsule toner is employed, because the toner surfaces are covered with a resin, there is substantially no surface for discharging, after the toner is charged through friction with a movable developer-carrying member. For this reason, the amount of charging of the toner cannot easily reach an equilibrium value within a short time, with the result that the image density of the copies obtained will disadvantageously be lowered with time.
Also, the microcapsule toner comprising cores having a dye or pigment dispersed in a soft binder or a low melting binder coated therearound with a hard resin is influenced by the soft binder or low melting binder core, even if surface-coated with a hard resin, and therefore inferior in free flowing property as compared with the toner obtained by the crushing method, because of stronger adhesive force between toners. Accordingly, the toner as a whole can hardly be subjected to uniform triboelectric charging through uniform friction with the movable developer-carrying member without irregularity. As the result, a part of the toner was insufficiently charged and liable to be attached onto the non-image portion of a resultant copy to cause a disadvantageous phenomenon of so-called fogging.